It is generally known that the movement and positioning of stepper motors can be precisely controlled. The rotational position of a magnetic rotor follows a magnetic field which is generated by driving phase shifted currents into a plurality of coils which are positioned around the rotor. If a stepper motor shall be used not only for a relative but as well for an absolute positioning of an object, at first a reference position has to be determined with respect to which the absolute position can be referenced. Then, a controlled absolute positioning is possible as long as the stepper motor is controlled under consideration of his characteristic movement parameters like the rotation angle, the velocity and the acceleration.
For determining a reference position, substantially two alternatives are known. These are on the one hand the mechanical reference run in which the motor is driven against a mechanical limit or stop which serves as a reference position and on the other hand the electric reference run in which a sensor (for example an electro mechanical switch or a light barrier) generates a related signal upon reaching the reference position.
Both alternatives have advantages and disadvantages. While the mechanical reference run may be connected with the development of noise and an increased wear due to the mechanical load, the realization of the electrical reference run is, due to the sensors, connected with higher cost, higher constructive requirements for the integration of the sensors into a mechatronic system and additional cabling requirements wherein especially under rough environment conditions the reliability of the sensors can create a problem.
Furthermore it has to be considered that by certain operating states like a suddenly occuring change of load due to an obstacle or similar, a loss of steps during a current operation of the stepper motor may occur or the stepper motor may even stop so that a new reference run becomes necessary. However, there are certain applications in which during a current operation of the motor a reference run is not possible so that besides the first time determination of the reference position as well a monitoring of the operating states of the stepper motor during the current operation—especially without any additional sensors—is desired.
EP 0 182 490 discloses a method for controlling a rotation detector circuit arrangement with which it is determined whether a driving pulse which is supplied to a stepper motor effects that the rotor is rotating in the correct direction. The rotation is substantially detected by connecting an element with a high and low impedance, respectively, electrically with a coil of the motor upon operating a first and a second circuit device, respectively, and by feeding first, second and third signals to the first and second circuit device in order to separate the element with the low impedance from the coil while the element with the high impedance is connected with the coil at the time of the detection of the rotation.
EP 1 017 159 discloses a method for controlling a voltage/frequency converter controlled single phase or polyphase electric motor with which a phase shift between an EMF voltage and a BEMF voltage is evaluated by way of the deviation between the zero crossing of the phase current and the voltage generated by the intrinsic induction and the frequency of the converter is accordingly readjusted. The measurement of the intrinsic induction is effected in this zero crossing of the current course of the associated phase, wherein during the measurement the phase is separated from the supply network.